This invention relates to derivatives of the heterocyclic system known as benzo[ij]quinolizine. More specifically it relates to 6,7-dihydro-1-oxo-1H,5H-benzo[ij]quinolizine-2-carboxylic acids and salts, hydrazides, amides and esters thereof. These compounds are optionally substituted at the 5, 6 or 7 positions by lower alkyl groups and on the benzo ring portion by a variety of substituents. The use of these compounds as antimicrobial agents and pharmaceutical compositions containing these compounds are also included within the present invention.